The number of SUVs and light trucks on the roads is currently increasing by about 15% per year. Part of the widespread popularity of these vehicles is their ability to operate under severe conditions, e.g., heavy towing of recreational equipment such as boats, or operating in rough and/or mountainous terrain. Owners expect to be able to use their vehicles for these purposes immediately, even in the first few hundred miles of the vehicle's life, which is characterized as the break-in period.
New gears contain surface imperfections that are inherent in the manufacturing process. During the break-in period, these imperfections are reduced through wear. The gears are worked which hardens and smooths the surface, thereby increasing the protection under slow speed and/or heavy load conditions where boundary conditions can exist. In the past, new gears were broken-in by the original equipment manufacturer (OEM). OEMs no longer do this, so, the break-in phase now occurs with the new vehicle owner. If the vehicle is placed under severe operating conditions, oil temperatures in the differential may increase well above 400° F. (about 200° C.), placing considerable strain on the new gears due to a thinning of the lubricant film that occurs on increasing temperature. This could result in damage to the differential in the form of heavy tooth spalling and breakage, bearing and seal failure, and ultimately, replacement of the entire differential.
Numerous efforts have been made to increase the temperature stability of lubricating oils. For instance, U.S. Pat. No. 5,354,484 teaches the use of at least one soluble tertiary aliphatic primary amine salt of a substituted phosphoric acid, and at least one soluble nitrogen- and boron-containing compound, in a lubricating composition said to be useful in gear applications requiring high thermal stability such as from about 160° C. with intermittent operation up to about 200° C.
U.S. Pat. No. 5,547,596 teaches a lubricant composition said to be useful for a limited slip differential of a car comprising a phosphate amine salt and borated ashless dispersant, the composition having a weight ratio of nitrogen to phosphorus ratio (N:P) of 0.5 or more, preferably 0.7 to 1.0, and a weight ratio of nitrogen to boron (N:B) of 4 to 10, preferably 6 to 9. Patentee states that if the ratio of N:B is more than 10, heat resistance deteriorates and sludge is liable to be generated.
U.S. Pat. No. 6,844,300 is directed to blending a base oil, a thermally stable phosphorus-containing anti-wear agent present in an amount sufficient to provide from 100 to 350 ppm phosphorus to the formulated gear and a metal-free sulfur-containing extreme-pressure agent present in an amount sufficient to provide at least 10,000 ppm sulfur to the formulated gear oil.
U.S. pplication No. 2004/0192564 is directed to a fluid to control temperature increase under trailer towing break-in conditions without prior conditioning of new (“green”) axles. The fluid is a bimodal gear lubricant producing a gel permeation chromatogram having at least two peaks. The first peak is representative of a base oil having a low viscosity of about 2 cSt to about 8 cSt and the second is representative of a viscosity index improver (VII) having a viscosity in the range of about 600 cSt to about 45,000 cSt at 100° C.
See also U.S. Patent Nos. 5,756,429; 5,801,130; EP 1422287 A1; and EP 531000 B1.
The present inventors have discovered a method of protecting new gears, and a composition suitable for practicing said method, by providing an additive system whereby in embodiments temperature increases in the oil lubricating the gears are lessened, even under harsh break-in conditions, and/or increased fuel economy is obtained in a vehicle having one or more parts lubricated by a composition comprising the additive system of the invention.